Abstract ：

利用定容燃烧弹研究了外加电场对甲烷-空气混合气燃烧特性的作用.加载电压为0、?5、?10和?12 kV,混合气的过量空气系数λ分别为0.8、1.0和1.6,代表浓燃、当量比和稀燃三种混合气状态.结果表明,电场方向上的火焰发展半径和火焰传播速度明显增加,原先近似圆球形的火焰形状发生变化.特别是稀燃混合气,电场作用下火焰的传播状况增加最为显著,火焰近似呈圆柱形.在加载电压为?12 kV时,对于λ=0.8、1.0和1.6的混合气,火焰拉伸速度最大值分别增加了42.3%、29.7%和111.7%.同时,混合气燃烧压力的发展明显加快,压力峰值出现的时间明显提前.对于浓燃和当量比浓度的混合气,压力峰值变化不大,而稀燃混合气的压力峰值增加较为明显.在?12 kV电压作用下,浓燃、当量比和稀燃三种状态下混合气的压力峰值时间提前了13.4%、7.5%和24.6%,相应的燃烧压力峰值增大了 2.2%、1.0%和 8.1%.本文应用外加电场对燃烧火焰产生的离子风效应及其对传播火焰形成的拉伸作用对试验结果进行了说明.

Keyword ：

传热 传质 电场 混合气 火焰传播速度 甲烷 离子风 燃烧压力

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Abstract ：

The charging time and the electrode gap distance are very important ignition parameters for the spark ignition process. The effects of these two parameters on the spark characteristics and the flame propagation of premixed methane-air mixtures were investigated experimentally at a constant pressure and a constant temperature by using a constant-volume bomb and a high-speed schlieren system. The parameters of the spark characteristics, including the spark voltage, the spark current, the spark duration and the lean burn ignition limit, were experimentally measured, and the spark resistance and the ignition energy were calculated on the basis of these parameters. By comparing the schlieren photographs of laminar flame propagation in different conditions taken by the high-speed schlieren devices, the effects of the ignition parameters on the cross-sectional area of the flame, the flame radius and the speed of flame propagation were analysed. The results show that the charging time and the electrode gap distance have important influences on the early kernel formation. Increasing the charging time or the electrode gap distance can significantly increase the influences of the effective radius of ignition and the original kernel size, can increase the ignition energy and can promote kernel development and flame propagation. Increasing the charging time or the electrode gap distance can effectively increase the lean burn ignition limit but, when the electrode gap distance increases to a certain extent, only increasing the charging time can increase the lean burn ignition limit further.

Keyword ：

constant-volume bomb flame propagation ignition parameters methane-air mixtures spark characteristics Spark ignition

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Abstract ：

We experimentally investigated the effects of DC electric field (U=10 kV) on flame propagation and combustion characteristics of premixed expanding CH4/O2/N2flames with equivalent ratio of 1.0 on the constant volume combustion bomb, so as to better understand the roles of electric field spatial distribution in the effects of electric field on flames.Five different electric field distributions will exist by changing voltage polarity of the two mesh electrodes, which are both negative(DN), both positive(DP), right positive and left negative(RPLN), right positive and left ground(RP) and left negative and right ground(LN). Flame front can be obviously stretched towards both the mesh electrodes by DN, DP and RPLN, while it can only be stretched towards one side by RP and LN where the electric field exists. The most effective electric field on flame propagation is DN, under which the normalized flame deformation ratio increases by 30.01% compared to that in the absence of electric field. The peak pressure time tpcan be advanced and both the initial combustion duration tidand main combustion duration tmdcan be shortened by DN, DP, RPLN, LN, and RP. RPLN affects tpmost obviously, and tpis advanced by 6.97%. The most effective electric field distribution on tidis DN which can shorten by 12.94%, and in turn is RPLN, LN, DP and RP. The top three of the most effective on tmdare LN, DN and DP, and tmdis shortened by 7.25%, 7.06%, and 6.49%, respectively. © 2016, High Voltage Engineering Editorial Department of CEPRI. All right reserved.

Keyword ：

Combustion duration Combustion pressure Deformation ratio Electric field distributions Premixed Propagation speed

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Abstract ：

In order to study the effect of the frequency and the intensity of low-frequency alternating-current (AC) electric fields on lean combustion, the effects of 100/80/60/40 Hz AC electric fields on premixed CH4/N-2/O-2 flames with excess air ratio of 1.2/1.4/1.6 have been investigated in a constant volume combustion bomb. The results show that the frequency and the applied voltage of low-frequency fields have significant effects on lean combustion. Firstly, the mean flame propagation speed decreases with the electric field frequency, and meanwhile, the combustion peak pressure decreases and the peak time increases. Secondly, both the mean flame propagation speed and the combustion peak pressure increase linearly with the applied voltage, while the peak time decreases. What's more, the developing degree of the ionic wind effect decreases with the electric field frequency and increases with the applied voltage, which can be used to explain the experimental results. The low-frequency AC electric fields can enhance the lean combustion effectively, which has a positive meaning in the further study of the electric field assisted combustion theory. (C) 2016 Elsevier Ltd. All rights reserved.

Keyword ：

AC electric field Bi-ionic wind effect Ionic wind effect Lean combustion

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Abstract ：

This paper compares numerical simulations with experiments to study the deformation of lean premixed spherically expanding flames under a negative direct current (DC) electric field. The experiments, including the flame deformation and the ionic distribution on the flame surface were investigated in a mesh to mesh electric field. Besides, a numerical model of adding an electric body force to the positive ions on the flame surface was also established to perform a relevant simulation. Results show that the spherical flame will acquire an elliptical shape with a marked flame stretch in the horizontal direction and a slight inhibition in the vertical direction under a negative DC electric field. Meanwhile, a non-uniform ionic distribution on the flame surface was also detected by the Langmuir probe. The simulation results from the numerical model show good agreement with experimental data. According to the velocity field analysis in simulation, it was found the particular motion of positive ions and neutral molecules on the flame surface should be responsible for the special flame deformation. When a negative DC electric field was applied, the majority of positive ions and colliding neutral molecules will form an ionic flow along the flame surface by a superposition of the electric field force and the aerodynamic drag. The ionic flow was not uniform and mainly formed on the upper and lower sides, so it will lead to a non-uniform ionic distribution along the flame surface. What's more, this ionic flow will also induce two vortexes both inside and outside of the flame surface due to viscosity effects. The external vortexes could produce an entraining effect on the premixed gas and take away the heat from the flame surface by forced convection, and then suppress the flame propagation in the vertical direction, while, the inner vortexes would scroll the burned zones and induce an inward flow at the horizontal center, which could be the reason for the pitted structure at the horizontal center when a high voltage was applied.

Keyword ：

electric field assisted combustion ionic distribution lean combustion numerical simulation spherically expanding flame

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Abstract ：

The ion current method was used to analyse effects of electric field on methane/air combustion at a constant volume bomb. The ion current as a characteristic parameter was obtained under different applied voltage (0∼-5kv). Combustion pressure was also regarded as another characteristic parameter. These two parameters were used to analyse effects of electric field on methane/air combustion. The results obtained by these two methods are exactly the same. Effects of electric field on combustion increase with the increase of the applied voltage; effects under lean combustion is most obvious, followed by rich combustion and equivalent ratio combustion. And the correlation coefficient of these two characteristic parameters is above 0.95.

Keyword ：

Applied voltages Combustion pressure Constant volume bombs Constant volume combustion bomb Correlation coefficient Effects of electric fields Equivalent ratios Ion currents

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Abstract ：

In order to study the effects of electric field intensity and distribution on flame propagation speed, the paper investigated the flame propagation of premixed CH4/O-2/N-2 mixtures under direct-current (DC) electric fields at an excess air ratio of 1.4. Different high-voltage electrodes were chosen in the experiment in two aspects: electrodes size (the outer-diameter of high-voltage electrodes) and electrodes structure. Results show that the flame propagation speed increases significantly with the voltage applied to the high-voltage electrodes. Secondly, the electric field intensity can significantly affect the flame propagation, and it can be observed that the mean flame propagation speed increases linearly with the mean electric field intensity under fixed high-voltage electrodes. In addition, electric field distribution is another important influence factor on the flame propagation. Specifically, the effect of the electric field on the flame propagation speed increases apparently with the uniformity coefficient, so a uniform electric field works better than an uneven one for promoting the flame propagation speed. In fact, with the smallest uniformity coefficient of 0.21, the electric field under the 60 mm-diameter mesh electrodes has the most uniform electric field, while the effect of that on the flame propagation is the strongest, with the highest mean flame propagation speed of 1.25 m/s at an applied voltage of -10 kV. The results substantiate the importance of the electric field intensity and distribution on promoting the flame propagation speed of lean combustion. (C) 2015 Elsevier Ltd. All rights reserved.

Keyword ：

Electric field intensity and distribution Electrodes size Electrodes structure Flame propagation speed Uniformity coefficient

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Abstract ：

In order to study the effect of the frequency and the intensity of high-frequency alternating-current (AC) electric fields on lean combustion, the effects of high-frequency (5-30 kHz; 5 kV) and high-voltage (0-5 kV; 15 kHz) AC electric fields on lean (excess air ratio 1.2/1.4/1.6) premixed CH4/N-2/O-2 flames are evaluated experimentally in the paper in detail. Results show that the mean flame propagation speed increases with the electric field frequency at first. However, the transition frequency, after which the mean flame propagation speed reaches to the maximum and then tends to be stabilized, is shown to be around 25 kHz. The combustion peak pressure increases minimally at various frequencies, but the timing of it decreases apparently; additionally, the effect of frequency on the combustion pressure change is very close to that on the mean flame propagation speed. The initial duration and main duration also have a similar tendency with the increase of the frequency. Besides, with the increase of the applied voltage, the mean flame propagation speed increases nearly exponentially. Meanwhile, the combustion peak pressure increases, and the timing of it decreases. Similarly, the effect of applied voltage on the combustion pressure change is also close to that on the mean flame propagation speed, and the initial duration and main duration also declines with the increase of the applied voltage. The result shows that the combustion of lean mixtures can be enhanced effectively by high-frequency (5-30 kHz) and high-voltage (1-5 kV) AC electric fields, which has a positive meaning in the further study of the electric-field-assisted combustion theory.

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Abstract ：

The ion current method was used to analyse effects of electric field on methane/air combustion at a constant volume bomb. The ion current as a characteristic parameter was obtained under different applied voltage (0∼-5kv). Combustion pressure was also regarded as another characteristic parameter. These two parameters were used to analyse effects of electric field on methane/air combustion. The results obtained by these two methods are exactly the same. Effects of electric field on combustion increase with the increase of the applied voltage; effects under lean combustion is most obvious, followed by rich combustion and equivalent ratio combustion. And the correlation coefficient of these two characteristic parameters is above 0.95.

Keyword ：

Bombs (ordnance) Combustion Correlation methods Electric fields Ions Methane

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Abstract ：

In this work, the effects of the electric fields on the flame propagation and combustion characteristics of lean premixed methane-air mixtures were experimentally investigated in a constant volume chamber. Results show that the flame front is remarkably stretched by the applied electric field, the stretched flame propagation velocity and the average flame propagation velocity are all accelerated significantly as the input voltage increases. This indicates that the applied electric field can augment the stretch in flame, and the result is more obvious for leaner mixture. According to the analyses of the combustion pressure variation and the heat release rate, the peak combustion pressure P-max increases and its appearance time t(p) is advanced with the increase of the input voltage. For the mixture of lambda = 1.6 at the input voltage of -12 kV, P-max increases by almost 12.3%, and t(p) is advanced by almost 31.4%, compared to the case of without electric fields. In addition, the normalized mass burning rate and the accumulated mass fraction burned are all enhanced substantially, and the flame development duration and the rapid burning duration are remarkably reduced with the increase of the input voltage, and again, the influence of electric field is more profound for leaner mixtures. The results can be explained by the electric field-induced stretch effects on lean burn methane-air mixture.

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